Will solar thermal heat up again?

Editor's Note: Scientific American's George Musser will be chronicling his experiences installing solar panels in Solar at Home (formerly 60-Second Solar). Read his introduction here and see all posts here.

It used to be that the term "solar panel" connoted a solar thermal panel, which uses sunlight to heat your house or tap water, as opposed to a photovoltaic (PV) panel, which produces electric power. These days, though, attention (not to mention sunlight) focuses on PV. Many people assume that solar hot water heaters are all well and good for, say, Israel, but ill-suited to high-latitude, cloudy, snowy climes such as the U.S. Northeast. But Scott Wilson of Olney, Md., begs to differ.

Responding to my call for stories about solar installations, Wilson sent me a description of his thermal system. His blog provides even more details and makes the case for installing thermal instead of photovoltaic panels:

"I don't have a grid-tied PV system, but I do have a solar thermal hot water system. As great as PV is, based on cost-efficiency, solar hot water should probably be most people's first contact with solar. I was on our local tour of solar homes this year, after saving up 1.5 years worth of performance data.

"Our system is a 16 tube evacuated array, with an equivalent power collecting ability of about 1.5 kW. It has a pressurized (34 psi) propylene glycol/water mixture that circulates through a manifold at the top of the array, picking up heat from a 'hot finger' that extends from the top of the individual tubes. That fluid travels to a heat exchanger mounted in the bottom of the tank, where it dumps the heat to the surrounding cold water, then travels back up to the roof. The water itself never travels to the roof, unlike the 'drain-back' flat plate systems of the '70s and '80s, which suffered, I'm told, from inferior quality issues, and gave solar domestic hot water a bad name.

"There are temperature sensors on the collector, which measures the maximum temperature directly from the tubes, and in the water at the bottom of the water tank. An electronic controller turns on a pump, which circulates the glycol fluid, when the temperature difference between the collector and the bottom of the tank exceeds a given amount, currently 10 degrees F. When the difference goes below 3 degrees F, the pump shuts off.

"The system basically preheats the water on the bottom of the water tank, so that the electrical element, mounted near the top of the tank, works less. In the summer, the water in the tank can be 120 degrees F at the top and as high as 110 degrees on the bottom, heated by the sun. I also added a small PV powered battery system for running the pump, so from about May to September, we turn off the electrical element at the breaker panel, and our entire hot water supply is off-grid.

"As to the cost, we paid $10,093.68 for the tubes, plumbing, pump, controller, data logger, new 120-gallon tank and installation. We received a $2,000 federal tax credit and a $2,000 Maryland state rebate. If you are in Montgomery County, Md., you also get a property tax credit of $1,500. So our net cost was $4,593.68. Our monthly savings, averaged over winter and summer for the past one-and-one-half years has been $30 per month. Our payoff will be 12.7 years. If you bought the same system now, you would still get a $2,000 state rebate, and a straight 30% federal tax credit, since the $2,000 cap has been lifted. Your net cost would be $3,565.58, including the Montgomery County credit. Your payoff period would be 9.9 years.

"As for our latitude, we bought the evacuated tube system over a flat plate system, since it is freeze-tolerant, and more efficient. But flat plat systems are sold here that also use glycol and are therefore freeze-tolerant also. An evacuated tube system like ours shouldn't really be limited by anything, other than available sun. On clear 20 degree days in winter, the tubes will get up to 70-75 degrees. And, unlike PV, shade doesn't completely destroy performance. I did a study comparing the energy collection in the fall and spring when the sun was at the same elevation, and the day length was the same. The only difference was leaves on the branches in the fall, and no leaves on branches in the spring. Leaves give about a 30% reduction versus no leaves, but I still collect energy when the sun penetrates leafless branches."

Some installers combine both types of panels in hybrid photovoltaic/thermal (PVT) systems, but they're not common. Indeed, some companies that used to install thermal systems, such as Borrego Solar, no longer do. Chris Anderson, Borrego's chief technology officer, says that thermal and PV systems require contractors with different skills and juggling the two would be tricky. That said, his own house has both. Someday, maybe all of ours will, too.

Scott Wilson's solar thermal array, courtesy of Scott Wilson

ABOUT THE AUTHOR(S)

George Musser

George Musser is a contributing editor for Scientific American, author of The Complete Idiot's Guide to String Theory (Alpha, 2008) and of Spooky Action at a Distance (Farrar, Straus and Giroux, 2015). This article won the 2011 Science Communication Award from the American Institute of Physics.

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